Mannich base condensation products which impart both viscosity index improving properties and dispersant properties to multigrade crankcase lubricants can be made by the condensation of oxidized olefinic polymer, formaldehyde (or formaldehyde precursor), and an amine. Processes for their preparation are taught in various patents (see, for example, U.S. Pat. No. 3,872,019 to Culbertson et al.).
The treatment of such Mannich base condensation products with oxygen (in air) to reduce their reactivity towards fluorocarbon elastomers used in engine seals is described in U.S. Pat. No. 4,455,244 to Smyser et al.
The treatment of such Mannich base condensation products with oxygen, inorganic peroxides, or percarboxylic acids to improve the fluorocarbon elastomer compatibility of a Mannich base dispersant is described in U.S. Pat. No. 4,379,064 to Cengel et al.
To the best of our knowledge however, nothing in the prior art has taught or suggested that the viscosity stability during storage of a Mannich base condensation product made with formaldehyde, a polyamine and an oxidized polyolefin could be improved by an oxidizing treatment with lower alkyl peroxides, lower alkyl hydroperoxides, or aryl(lower)alkyl hydroperoxides.